Self-oscillating converters typically depend upon the equivalent series resistance (ESR) of the output or load capacitor for their operation. Modern capacitors, such as, for example, small high-frequency ceramic thin film capacitors, have a negligible ESR. Then, an external resistance is needed. But the external resistance leads to a higher output ripple waveform of the output voltage.
With no or too small an equivalent series resistance, the oscillator becomes unstable or only conditionally stable with, for example, more than one stable oscillation frequency.